According to the present invention, exhaust components such as exhaust manifolds, include multiple components formed from a combination of different materials. The use of different materials allows a low cost material (typically with lower temperature capability) to be used in regions of the component that are generally subjected to lower temperatures and less severe loading, and a higher grade, more temperature resistant material, in regions of high temperature, high thermal gradients, or high stress.
One typical example would involve a cast iron exhaust manifold. It is common to employ alloyed (e.g. with silicon and molybdenum) cast iron exhaust manifolds on high volume production engines because they often have advantages in terms of cost, durability, packaging, and NVH (noise, vibration, and harshness). Packaging refers to the task of arranging flow paths from each port to a common outlet position (with minimum flow losses) while maintaining clearance to other underhood components and providing access for all fasteners during assembly. However, as automotive companies increase the gas temperatures of their engines to improve efficiency and reduce exhaust emissions, manifold applications are exceeding the practical working (temperature) limit of the cast iron employed. One factor contributing to the working temperature range of a ferritic material is the AC1 temperature. The AC1 temperature marks the beginning of a transformation from the body-centered-cubic molecular structure associated with ferrite to the face-centered-cubic structure of an austenitic material. Many things occur with the transformation from ferrite to austenite, including a marked change in solubility of certain alloying elements. At temperatures above the AC1, the cast iron is more prone to damage from oxidation, decarburization, and coarsening. This means that in applications where the metal temperatures are above the AC1, there is much greater propensity to incur material damage. Higher temperature and longer duration at elevated temperature will result in greater material damage accumulation.
Upon closer inspection of exhaust manifolds after extended use at high temperature, it becomes evident that heat transfer is non-uniform and that certain areas of the manifold are much hotter than other areas. SiMo (silicon-molybdenum) cast iron exhaust manifolds have an AC1 temperature of approximately 830-840° C. Since a typical maximum manifold outlet gas temperature for a current North American gasoline engine is about 900° C., it can be shown that most areas of the manifold will be below the AC1 temperature. Currently, if a material such as SiMo cast iron, for example, is inadequate for the peak temperature areas, the entire manifold has to be made in a higher grade material (Ni-Resist, cast steel, or fabricated from stainless steel). In view of the foregoing, the inventors of the present invention suggest that exhaust components, including a divider plate assembly, can be made of a combination of materials, using the high temperature material where required and the lower temperature material elsewhere.
The discussion above focuses mainly on absolute temperature and the general resistance to damage of a material subjected to those temperatures. An additional application for composite exhaust components exists when the mechanical loads and/or thermally induced strains are too great for certain materials. Even if the temperature is below the AC1 temperature, regions that are subjected to high thermal strains and/or mechanical loads can fail prematurely due to cyclic thermal mechanical fatigue. Areas that are prone to extreme thermal gradients are common such as shared walls between exhaust runners or bifurcations that separate plenums/chambers/runners in the manifold. These areas have high heating and cooling thermal gradients because they are simultaneously heated (or cooled, depending on the mode of operation of the test cycle) from both sides.
Referring particularly to exhaust components such as manifolds, the highest (steady state) material temperatures are generally in the region of the manifold outlet, i.e. the area in which the manifold runners leading from the engine block are joined. If the exhaust flow is separated by a bifurcation or shared wall, the thermal gradients (which cause local strains) are greatest in this region because of the (transient) heating and cooling from both sides. Replacing the low temperature material in the critical regions (bifurcation, shared wall, outlet region) with a material more appropriate for the local loading and temperature requirements would result in a more cost effective solution than upgrading the material of the entire manifold. More particularly, the present invention provides divider plate assemblies for location in high temperature areas such as exhaust manifold outlets, wherein at least one of the components of the divider plate assembly is formed from a high temperature capable material. Such “high temperature” materials refer to materials with the desired material properties at elevated temperatures such as strength, microstructural stability, and/or oxidation resistance, by way of non-limiting examples.
Single material cast exhaust components can suffer severe damage in regions of local high temperature and large thermal gradients such as the outlet or a bifurcation. The high temperature promotes oxidation and the thermal gradients introduce local strains that may make the oxide layer less adherent. If spalling of the oxide occurs, particles are released into the exhaust gas stream that may bombard and damage downstream components such as turbochargers and catalytic converters. Exhaust components incorporating divider plate assemblies, such as those described herein, reduce or eliminate this mechanism and in turn maintain system performance, rather than causing it to degrade with time.
The oxidation, particle coarsening, and decarburization that occurs locally in the high temperature regions can significantly degrade the local material properties over time. Obviously this can result in premature cracking and warpage, both of which can reduce component durability performance. These effects, in turn, can result in exhaust gases leaking to the environment (through a crack or loss of sealing) or allow exhaust gas to communicate (travel) between separated runners or chambers (either will negatively influence system performance). If large thermally induced strains are co-located with the manifold areas with degraded material properties, component failure by cracking is common.
In addition, use of a divider plate assembly may result in reduced loading caused by the divider/bifurcation on the surrounding manifold material. One example of this is when the divider plate is made of a high temperature capable material, and the plate is designed to flex or elastically deform during rapid heating. This helps to reduce the loads that are transferred from the plate to the surrounding structures. Another method of reducing the loads occurs when there is sliding contact between the insert plate and the main manifold body. This arrangement occurs with the slide-in insert, and possibly with the cast-in insert plate, when the plate is made from a ceramic or covered in a thin refractory coating or other suitable material prior to casting the manifold. With a small gap between the insert plate and the body, differential expansion/contraction is possible without transferring large loads between the two members.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.